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Increasing population and employment density in metropolitan areas could reduce vehicle travel, energy use, and CO2 emissions from less than 1 percent up to 11 percent by 2050 compared to a base case for household vehicle usage, says a new congressionally mandated report from the National Research Council, although committee members disagreed about the plausibility of achieving the higher estimate. Assuming compact development is focused on new and replacement housing -- as converting existing housing to higher densities could be prohibitively difficult -- significant increases in density would result in modest short-term reductions in personal travel, energy use, and CO2 emissions. However, these reductions will grow over time.

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Currently, 80 percent of Americans live in metropolitan areas, but population and employment are increasingly decentralized. This trend of suburbanization, made possible largely due to automobiles and extensive highway systems, reflects the preferences of many Americans for living in detached, single-family homes. Dispersed, automobile-dependent development patterns, however, involve numerous costs: the use of vast quantities of land, increased reliance on petroleum, and increased greenhouse gas emissions. Compact, mixed-use development -- individuals living in denser environments with jobs and shopping close by -- could reduce the number of vehicle miles traveled by shortening trip lengths, the report says, and by making walking, biking, and public transit more viable alternatives to driving.

According to the committee that wrote the report, the most reliable research studies estimate that doubling residential density in a metropolitan area might lower household driving between 5 percent and 12 percent. If higher density were paired with more concentrated employment and commercial locations, and combined with improvements to public transit and other strategies to reduce automobile travel, household driving could be lowered by as much as 25 percent. By reducing vehicle use, petroleum use and CO2 emissions would also be lessened.

In order to quantify the potential effects of compact development, the committee developed illustrative scenarios, looking forward to 2030 and 2050. If 75 percent of new and replacement housing units in the U.S. were developed at twice the density of current new development, and individuals drove 25 percent less -- the committee's upper-bound scenario -- personal travel, fuel use, and CO2 emissions would be reduced by 7 percent to 8 percent, relative to a base case, by 2030, and by 8 percent to 11 percent by 2050. If only 25 percent of housing units were developed more compactly, and residents drove 12 percent less, then personal travel, fuel use, and CO2 emissions would be reduced by approximately 1 percent by 2030, and by 1.3 percent to 1.7 percent by 2050. If in this lower-bound scenario residents drove only 5 percent less, then personal travel, fuel use, and CO2 emissions would be reduced by less than 1 percent by 2050.

The committee disagreed about the feasibility of achieving the target density in the upper-bound scenario -- doubling the density of 75 percent of new development -- by 2050. Some members of the committee thought that these higher densities would be reached due to macroeconomic trends -- higher energy prices and carbon taxes -- in combination with growing public support for infill development, investments in transit, and higher densities along transit rail corridors. Other members thought that the high-density scenario would require such a significant departure from current low-density development patterns, land-use policies, and public preferences that it is unrealistic without a strong state or regional role in growth management.

A number of obstacles stand in the way of widespread compact development in the U.S. States and regional entities would need to take a major role in managing local growth, which is now controlled by local governments. Local zoning regulations would be a large obstacle, and compact-development initiatives could meet resistance from existing homeowners and politicians. Their legitimate concerns about congestion, local taxes, or home values may be at odds with regional and national goals, such as housing affordability or climate change. In the near term, the primary opportunities to increase density are in areas already experiencing such changes, like the inner suburbs and areas close to public transit or along major highway corridors. Over the long term, adopting compact development would likely require changes in housing preferences and a greater political consensus in support of stronger state and regional control of land use. Public infrastructure investments, such as public transit, and market-based strategies like congestion pricing and higher parking fees, could be a way to steer communities toward compact, mixed-use development, but, in the case of tranist, would require significant new investments.

Government policies to support more compact, mixed-use development should be encouraged, the report says. The nation is likely to set ambitious goals to address climate change and, given the large contribution of the transportation sector to greenhouse gas emissions, changes in land use may have to be part of the effort. If so, land use changes should be implemented soon, because current development patterns will take decades to reverse. Nevertheless, the committee recognizes that it does not have as much verifiable scientific evidence to support this recommendation as it would like. It lacks a more complete understanding of how specific land-use policies might affect travel in different metropolitan areas and a fuller accounting of the costs and benefits of compact, mixed-use development. Given these limitations, the committee urges that it would be wise to proceed carefully, monitoring the results and incorporating new research as it becomes available.

The study was sponsored by the U.S. Department of Energy. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council are private, nonprofit institutions that provide science, technology, and health policy advice under a congressional charter. The Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. A committee roster follows.

National Academy of Sciences. (2009, September 2). Increasing Residential And Employment Density Could Mean Reductions In Vehicle Travel, Fuel Use And Carbon Dioxide Emissions. ScienceDaily. Retrieved March 31, 2015 from www.sciencedaily.com/releases/2009/09/090901104854.htm

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